Interesting question that has been thrashed around in these forums a couple of times. The bribe cost is a complex calculation involving the size of the treasury of the civ you're bribing, the city's distance from the capital, whether or not the city is in disorder, whether there are units in the city, and whether the bribing spy is a vet. It's also cheaper to bribe a city that was once your own - which is to say, the other civ took it from you at some stage.
The bigger the civ's treasury, the dearer the city will be; the further the city is from the capital, the cheaper it should be; revolt in a city at the time of bribery reduces the cost by 50%; the cost of an undefended city drops by 50%; and bribery by a vet dip is cheaper than bribery by a non-vet dip.
Someone even discovered that approaching (and bribing) a city from different directions - N,S,E or W - can vary the cost of the bribe.
There has been considerable debate about the actual equation used by the software and the mathematicians amongst us have torn out their hair trying to analyse it.
Hope this helps.
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